The need for an individual automatic temperature control in central-heated structures is felt to an increasing degree with the continued increase in energy costs. In central heating systems an individual thermostatic control of the different radiators has previously not been feasible. The prior known thermostat units require, for installation, rather extensive changes of the radiator connections, and since all the radiators in a building must be equipped with thermostat units if the full advantage of thermostatic control is to be attained, the necessary installation work may well be too extensive and costly to be deemed justifiable and compensated for by future savings.